The ability to maintain communications not withstanding network failures has become increasingly important. Fiber facility cuts, central office failures, and equipment failures are the failure mechanisms of most concern to network providers. Such failures, which have disrupted communications service for days, are catastrophic to information intensive industries that rely on communications to accomplish their day-to-day business.
Prior attempts to avoid a communications interruption caused by network failures in a primary central office serving a subscriber (customer), in the communications link between a customer and the primary central office or in the link between the primary central office and a hub office have resulted in undesirable consequences. These consequences, which included excessive switching, failure propagation, and unnecessary maintenance activity, are brought about because of ineffective switching architectures and ineffective switch triggering mechanisms. In addition, prior switching architectures could not survive a failure of communications between the customer location and the primary central office, and a simultaneous failure of communications between the primary central office and the corresponding hub office.